The present invention relates to a cathode ray tube having an electron gun and a device for magnetic focusing of the electron beam issuing from said gun.
Magnetic focusing units are normally intended for use with cathode ray tubes of high resolution. Owing to the creation of a magnetic field coaxial with the electron beam issuing from the gun, they permit making the impact of this beam on the screen of the tube as small as desired.
Such tubes are used, for example, in the field of television by projection or professional display devices of high resolution. The focusing lens created by the magnetic focusing device can be realized either by means of a permanent magnet or by means of a circular winding enclosed in a magnetic circuit having a gap in which a possibly adjustable direct current flows.
It may be necessary to add a dynamic focusing coil to the main focusing coil in order to focus the electron beam over the entire surface of the screen of the tube with the same precision. In that case, the field created by the dynamic focusing coil will modulate the field of the main focusing coil depending on the position of the point of impact of the beam on the screen. The frequency of the current inside the auxiliary coil is the same as the line scan frequency of the screen. This presents certain problems at high scanning frequencies, for example, on the order of 16 kHz or higher, including:
(1) strong electric coupling between the main static coil and the dynamic coil;
(2) strong current, induced by the dynamic focusing coil, in the frame of the magnetic circuit containing the static focusing coil, hence a loss of energy;
(3) appearance of magnetic drag, due to the time for establishment of the field in the magnetic circuit, becoming critical at high scanning frequencies; and,
(4) ever weaker action of the dynamic field on the electron beam as the scanning frequency increases.